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arXiv:2010.08783 · Physical Review Applied

Passive Nonlinear Optical Isolators Bypassing Dynamic Reciprocity

Yiqi Hu, Yihong Qi, Yu You, Shicheng Zhang, Gongwei Lin, Xiaolin Li, Jiangbin Gong, Shangqing Gong, Yueping Niu

Published Jul 19, 2021• 35 citations

Abstract

Magnetic-free optical isolators are critical components for the realization of integrated optical systems. The underlying physics of passive nonlinear optical isolators is not solely about breaking the Lorentz reciprocity without requiring any external bias. Indeed, one major obstacle to the operation of self-Kerr-type nonlinear optical isolators is the so-called dynamic reciprocity, of which a serious outcome is that a backward signal cannot be isolated in the presence of a forward strong signal. In this work, we advocate the concept of velocity-selective nonlinearity to bypass such dynamic reciprocity. Using a proof-of-principle platform with warm rubidium atoms in an asymmetric cavity, we experimentally achieve the isolation of a backward signal in the presence of a strong forward signal, with experimental observations in agreement with our theoretical simulations. This work has thus made one essential step towards functioning Kerr-type passive optical isolators.

DOI: 10.1103/physrevapplied.16.014046

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Magnetic-free optical isolators are critical components for the realization of integrated optical systems. The underlying physics of passive nonlinear optical isolators is not solely about breaking the Lorentz reciprocity without requiring any external bias. Indeed, one major obstacle to the operation of self-Kerr-type nonlinear optical isolators is the so-called dynamic reciprocity, of which a serious outcome is that a backward signal cannot be isolated in the presence of a forward strong signal. In this work, we advocate the concept of velocity-selective nonlinearity to bypass such dynamic reciprocity. Using a proof-of-principle platform with warm rubidium atoms in an asymmetric cavity, we experimentally achieve the isolation of a backward signal in the presence of a strong forward signal, with experimental observations in agreement with our theoretical simulations. This work has thus made one essential step towards functioning Kerr-type passive optical isolators.

On-chip optical nonreciprocal devices are vital components for integrated photonic systems and scalable quantum information processing. Nonlinear optical isolators and circulators have attracted considerable attention because of their fundamental interest and their important advantages in integrated photonic circuits. However, optical nonreciprocal devices based on Kerr or Kerr-like nonlinearity are subject to dynamical reciprocity when the forward and backward signals coexist simultaneously in a nonlinear system. Here, we theoretically propose a method for realizing on-chip nonlinear isolators and circulators with dynamic nonreciprocity. Dynamic nonreciprocity is achieved via the chiral modulation on the resonance frequency due to coexisting self- and cross-Kerr nonlinearities in an optical ring resonator. This work showing dynamic nonreciprocity with a Kerr nonlinear resonator can be an essential step toward integrated optical isolation.

Achieving a broadband nonreciprocal device without gain and any external bias is very challenging and highly desirable for modern photonic technologies and quantum networks. Here we theoretically propose a passive and magnetic-free all-optical isolator for a femtosecond laser pulse by exploiting a new mechanism of unidirectional self-induced transparency, obtained with a nonlinear medium followed by a normal absorbing medium at one side. The transmission contrast between the forward and backward directions can reach 14.3 dB for a 2π - 5 fs laser pulse. The 20 dB bandwidth is about 56 nm, already comparable with a magneto-optical isolator. This work provides a new mechanism which may benefit non-magnetic isolation of ultrashort laser pulses.

Abstract Current commercial optical isolators (OIs) primarily rely on the Faraday effect in magneto-optical materials, which necessitates the use of external magnetic fields. Here, we report a passive OI constructed from compact, commercially available optical components and an atomic vapor cell, which does not require a biasing magnetic field or any external drive, and only requires that the intensity of the forward beam is higher than that of the backward beam, which is very easy to satisfy in isolator applications. Operating at 795 nm, this device uses the principle of nonlinear non-reciprocity to achieve a bandwidth of 6 GHz and an isolation ratio greater than 30 dB.

Optical isolation is important for protecting a laser from damage due to the detrimental back reflection of light. It typically relies on breaking Lorentz reciprocity and normally is achieved via the Faraday magneto-optical effect, requiring a strong external magnetic field. Single-photon isolation, the quantum counterpart of optical isolation, is the key functional component in quantum information processing, but its realization is challenging. In this chapter, we present all-optical schemes for isolating the backscattering from single photons. In the first scheme, we show the single-photon isolation can be realized by using a chiral quantum optical system, in which a quantum emitter asymmetrically couples to nanowaveguide modes or whispering-gallery modes with high optical chirality. Secondly, we propose a chiral optical Kerr nonlinearity to bypass the so-called dynamical reciprocity in nonlinear optics and then achieve room-temperature photon isolation with low insertion loss. The concepts we present may pave the way for quantum information processing in an unconventional way.

Optical nonlinearity has been widely used to try to produce optical isolators. However, this is very difficult to achieve due to dynamical reciprocity. Here, we show the use of the chiral cross-Kerr nonlinearity of atoms at room temperature to realize optical isolation, circumventing dynamical reciprocity. In our approach, the chiral cross-Kerr nonlinearity is induced by the thermal motion of N-type atoms. The resulting cross phase shift and absorption of a weak probe field are dependent on its propagation direction. This proposed optical isolator can achieve more than 30 dB of isolation ratio, with a low loss of less than 1 dB. By inserting this atomic medium in a Mach-Zehnder interferometer, we further propose a four-port optical circulator with a fidelity larger than 0.9 and an average insertion loss less than 1.6 dB. Using atomic vapor embedded in an on-chip waveguide, our method may provide chip-compatible optical isolation at the single-photon level of a probe field.